Routing on One Interface

TomEastep

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Caution

This article applies to Shorewall 4.3 and
later. If you are running a version of Shorewall earlier than Shorewall
4.3.5 then please see the documentation for that
release.

Introduction

While most configurations can be handled with each of the firewall's
network interfaces assigned to a single zone, there are cases where you
will want to divide the hosts accessed through an interface between two or
more zones.

You are using some form of NAT and want to access a server by
its external IP address from the same LAN segment. This is covered in
FAQs 2 and 2a.

There are routers accessible through the interface and you want
to treat the networks accessed through that router as a separate
zone.

Some of the hosts accessed through an interface have
significantly different firewalling requirements from the others so
you want to assign them to a different zone.

The key points to keep in mind when setting up multiple zones per
interface are:

Shorewall generates rules for zones in the order that the zone
declarations appear in /etc/shorewall/zones unless you modify the
processing order using the explicit
child-zone:parent-zone
syntax, in which case the child zone rules are generated first.

The order of entries in /etc/shorewall/hosts is immaterial as
far as the generated rule set is concerned.

These examples use the local zone but the same
technique works for any zone. Remember that Shorewall doesn't
have any conceptual knowledge of “Internet”,
“Local”, or “DMZ” so all zones except the
firewall itself ($FW) are the same as far as Shorewall is concerned. Also,
the examples use private (RFC 1918) addresses but public IP addresses can
be used in exactly the same way.

Router in the Local Zone

Here is an example of a router in the local zone.

Note

the box called “Router” could
be a VPN server or other such device; from the point of view
of this discussion, it makes no difference.

Can You Use the Standard Configuration?

The firewall requirements to/from the Internet are the same
for 192.168.1.0/24 and 192.168.2.0/24.

The hosts in 192.168.1.0/24 know that the route to
192.168.2.0/24 is through the router.

All you have to do on the firewall is add a route to
192.168.2.0/24 through the router and
restart Shorewall.

Will One Zone be Enough?

If the firewalling requirements for the two local networks is the
same but the hosts in 192.168.1.0/24 don't know how to route to
192.168.2.0/24 then you need to configure the firewall slightly
differently. This type of configuration is rather stupid from an IP
networking point of view but it is sometimes necessary because you
simply don't want to have to reconfigure all of the hosts in
192.168.1.0/24 to add a persistent route to 192.168.2.0/24. On the
firewall:

Add a route to 192.168.2.0/24 through the Router.

Set the “routeback” option for eth1 (the local
firewall interface) in /etc/shorewall/interfaces.

Restart Shorewall.

I Need Separate Zones

If you need to make 192.168.2.0/24 into its own zone, you can do
it one of two ways; Nested Zones or Parallel Zones.

Nested Zones

You can define one zone (called it “loc”) as being
all hosts connected to eth1 and a second zone “loc1”
(192.168.2.0/24) as a sub-zone.

Note

The Router in the above diagram is assumed to NOT be doing
SNAT for the hosts in the 192.168.2.0/24 network.

The advantage of this approach is that the zone
“loc1” can use CONTINUE policies such that if a
connection request doesn't match a “loc1” rule, it will
be matched against the “loc” rules. For example, if your
loc1->net policy is CONTINUE then if a connection request from loc1
to the Internet doesn't match any rules for loc1->net then it will
be checked against the loc->net rules.

/etc/shorewall/zones

#ZONE TYPE OPTIONS
loc ipv4
loc1:loc ipv4

Note

the Parent zone (loc) is defined first then the sub-zone
(loc1) is defined using the special syntax shown.

/etc/shorewall/interfaces

#ZONE INTERFACE OPTIONS
loc eth1 -

/etc/shorewall/hosts

#ZONE HOSTS OPTIONS
loc1 eth1:192.168.2.0/24 -

If you don't need Shorewall to set up infrastructure to route
traffic between “loc” and “loc1”, add these
two policies.

/etc/shorewall/policy

#SOURCE DEST POLICY
loc loc1 NONE
loc1 loc NONE

Parallel Zones

You define both zones in the /etc/shorewall/hosts file to create
two disjoint zones.

Note

The Router in the above diagram is assumed to NOT be doing
SNAT for the hosts in the 192.168.2.0/24 network.

You don't need Shorewall to set up infrastructure to route
traffic between “loc” and “loc1”, so add
these two policies:

#SOURCE DEST POLICY
loc1 loc2 NONE
loc2 loc1 NONE

Some Hosts have Special Firewalling Requirements

There are cases where a subset of the addresses associated with an
interface need special handling. Here's an example.

In this example, addresses 192.168.1.8 - 192.168.1.15
(192.168.1.8/29) are to be treated as their own zone (loc1).

/etc/shorewall/zones

#ZONE TYPE OPTIONS
loc ipv4
loc1:loc ipv4

/etc/shorewall/interfaces

#ZONE INTERFACE
loc eth1

/etc/shorewall/hosts

#ZONE HOSTS OPTIONS
loc1 eth1:192.168.1.8/29 broadcast

The broadcast option causes limited broadcasts
(destination IP address 255.255.255.255) to be checked against the loc1
zone. If that zone allows the packet, then of course it will be seen by
the mempers of loc that are outside of loc1 as well. If the broadcast is
not specifically DROPped by the fw->loc1 rules then it will also be
checked against the fw->loc rules, even if there is a DROP or REJECT
fw->loc1 policy.

You probably don't want Shorewall to set up infrastructure to route
traffic between “loc” and “loc1” so you should
add these two policies.

/etc/shorewall/policy

#SOURCE DEST POLICY
loc loc1 NONE
loc1 loc NONE

One-armed Router

Nested zones may also be used to configure a
“one-armed” router (I don't call it a “firewall”
because it is very insecure. For example, if you connect to the Internet
via cable modem, your next door neighbor has full access to your local
systems as does everyone else connected to the same cable modem head-end
controller). Here eth0 is configured with both a public IP address and an
RFC 1918 address (More on that topic may be found here). Hosts in the
“loc” zone are configured with their default gateway set to
the Shorewall router's RFC1918 address.

/etc/shorewall/zones

#ZONE TYPE OPTIONS
net ipv4
loc:net ipv4

/etc/shorewall/interfaces

#ZONE INTERFACE OPTIONS
net eth0 routefilter

/etc/shorewall/hosts

#ZONE HOSTS OPTIONS
loc eth0:192.168.1.0/24 maclist

/etc/shorewall/masq

#INTERFACE SOURCE ADDRESS
eth0:!192.168.1.0/24 192.168.1.0/24

Note that the maclist option is
specified in /etc/shorewall/interfaces. This is to
help protect your router from unauthorized access by your friends and
neighbors. Start without maclist then add
it and configure your /etc/shorewall/maclist
file when everything else is working.